Answer: D. An invocable method

A screen flow can call Apex through an Apex action, which runs a method annotated with @InvocableMethod. The method makes the callout to the external order management system in real time and returns the order numbers and dates. The flow then shows them on a screen, for example in a data table or a choice list.

apex
public class GetExternalOrders {
public class Request { @InvocableVariable public String customerId; }
public class OrderInfo {
@InvocableVariable public String orderNumber;
@InvocableVariable public Date orderDate;
}

@InvocableMethod(label='Get external orders' callout=true)
public static List<List<OrderInfo>> getOrders(List<Request> requests) {
// HTTP callout to the order system, then build and return the results
}
}

The callout=true setting is needed so the flow can make the callout.

The other options don’t fit:

A. Outbound message: It pushes data from Salesforce to an external system when a workflow rule fires. It can’t retrieve data on demand.
B. Apex REST class: It exposes Salesforce as a web service that external systems call into. It is for inbound requests, not for calling out to another system.
C. Apex controller: It backs Visualforce pages and Lightning components. A flow can’t call it directly, and only methods exposed as invocable actions are available in Flow....

Answer: D. An invocable method

A screen flow can call Apex through an Apex action, which runs a method annotated with @InvocableMethod. The method makes the callout to the external order management system in real time and returns the order numbers and dates. The flow then shows them on a screen, for example in a data table or a choice list.

apex
public class GetExternalOrders {
public class Request { @InvocableVariable public String customerId; }
public class OrderInfo {
@InvocableVariable public String orderNumber;
@InvocableVariable public Date orderDate;
}

@InvocableMethod(label='Get external orders' callout=true)
public static List<List<OrderInfo>> getOrders(List<Request> requests) {
// HTTP callout to the order system, then build and return the results
}
}

The callout=true setting is needed so the flow can make the callout.

The other options don’t fit:

A. Outbound message: It pushes data from Salesforce to an external system when a workflow rule fires. It can’t retrieve data on demand.
B. Apex REST class: It exposes Salesforce as a web service that external systems call into. It is for inbound requests, not for calling out to another system.
C. Apex controller: It backs Visualforce pages and Lightning components. A flow can’t call it directly, and only methods exposed as invocable actions are available in Flow....